Whoa that's a very hard question... But what are you talking about the PC or Xbox systems or what? Because I have never Heard of "Halo 3" on PC. But here is what my favorites are on gaming system and PC
Gaming System : Graft Thief Auto 4 PC : MapleStory
Mine is playing DotA in Warcraft III Frozen Throne for PC. Other one is Grand Theft Auto San Andreas also for PC. And playing Dawn of War [I forgot what DoW...] for PC and playing Chrono Trigger and Cross for my old PS1! Hmmm, that's about it. [I also play this racing game, but I forgot the title, it's a cool street-racing game for me!]
Best game on PC would have to be...Age of Empires 3 Best game on PS2 would have to be...Lord of the Rings 3 Best game on Xbox360...The Elder Scrolls IV: Oblivion Best game overall: OBLIVION@!!! :-)
why just computer? Oh all right, you won't like it though. Lord of the Rings: Battle for Middle Earth 2.
Best game I've ever played for anything (I have a wide range of played games) would have to be: The Elder Scrolls IV: OBLIVION! Seriously, buy it. If you have a 360, buy it for that. If not, get it for PC!
The pc game Ive played most would be WarHammer Dawn of War and its expansions, so that I would say would be my Fav. But i do remember a few years ago SWAT came out for pc thats also a good game, off subject but i just remembered it for some odd reason.